The present invention relates to an improved apparatus and method for cutting deep slots in earth formations. More specifically, the present invention relates to an apparatus and method for cutting deep slots in earth formations to aid in the mining of mineral deposits, particularly coal. Still more specifically, the present invention relates to an apparatus and method for cutting deep, slightly-inclined slots beneath overburden, in aid of the movement of massive, substantially unbroken blocks of such overburden, particularly in the mining of shallow coal deposits.
In present day techniques for mining near surface deposits of coal and the like, the body of earth or overburden located on top of the mineral deposit is first removed. This removal of the overburden may be accomplished by various methods of excavation, but the basic operation consists of stripping the overburden to expose the mineral and depositing the stripped overburden in a mound in the area previously mined. The removal of the overburden, in strip mining operations, is generally performed by scraper-type earth-moving machines or bucket-type cranes. Thus, the major expense in strip mining is the excavation of large volumes of overburden materials.
In addition, special precautions must be taken with regard to toxic wastes, which, until the recent enforcement of stricter regulations, were often brought to the surface and dumped along with the overburden materials. Sulfuric acid, a product of sulfide weathering, and silt were then washed into the surface waters of the surrounding countryside.
It is therefore desirable to provide a mining technique for the removal of near surface mineral deposits which is less expensive than present day strip mining which can be carried out with a minimum of disturbance to the surface of the earth, and which minimizes the deposition of deleterious materials on or near the surface of the earth.
A mining technique which accomplishes these objectives is set forth in Applicant's U.S. Pat. No. 3,917,349. U.S. Pat. No. 3,917,349 describes a method for exposing sedimentary mineral deposits, such as coal, by shifting massive, substantially unbroken blocks of overburden on a Bingham plastic lubricant film, such as a clay-water paste. Each overburden block is prepared for movement by separating it from adjacent material along vertical faces by trenching and by pressure induced fractures. A basal plane of separation is formed above the mineral deposit by hydraulic fracturing along a bedding plane or by mechanically cutting a slot. The clay paste lubricant is injected into this basal plane of separation and also deposited on an earth fill ramp in front of the block. When the block is released by fracturing along its rear face, it slides onto the fill ramp covering the pit in the mined out area in front of the block and simultaneously opens a new pit to the rear of the block from which the mineral deposit can then be removed. The pit into which the block moves has been formed by the movement of a previous block, except for a starting excavation which provides the space for the movement of the first block of a series. Generally, the blocks will be impelled down slope by gravity. Each block movement is followed by removal of the mineral deposit from the bottom of the newly formed pit, the grading of a new ramp of fill in the bottom of the pit and the preparation of the next block for movement.
While separating the block of overburden along its base may be accomplished by hydraulic fracturing or undercutting, as disclosed in prior U.S. Pat. No. 3,917,349, undercutting is the preferred technique, since the inclination and flatness of the plane of separation can then be controlled. However, apparatus does not presently exist for mechanically cutting slots of the configuration necessary to efficiently practice the method of U.S. Pat. No. 3,917,349 (for example, 2 to 6 inches in thickness and about 100 feet deep from the exposed highwall to the rear of the block).
The object of the present invention is to provide such an undercutting apparatus. A further object is to provide an improved method for clay injection during the undercutting operation. A still further object of this invention is to provide an improved overburden block lubrication system for carrying out the mining method of U.S. Pat. No. 3,917,349. More specific objects of the present invention are to provide a simple powerful control system for guiding the cutting direction of the undercutting tool, to provide an efficient system for injecting high gel strength clay paste into the slot at the optimum entry point, to provide a sealing system which minimizes the unsupported portion of the slot, to provide a means for establishing an improved system for lubricating the block of overburden, which minimizes the strains to which the overburden is subjected, and to provide a system which will lubricate the block of overburden, despite the absence of substantial strength and rigidity in the block itself. A further object of the present invention is to provide a means for mining thin seams of coal.
In accordance with the present invention, a deep slot, having a large depth to thickness ratio is cut in an earth formation by a cutting assembly extending lengthwise to the full depth of the slot with its forward portion along the edge of the slot being cut. The cutting assembly includes a support extending to the full depth of the slot; a cutter bar extending along the forward portion of the support; a passage incorporated in the support for transmitting a liquid along the support, discharging the liquid in the interior of the slot in an amount and under a pressure sufficient to resist the earth pressure tending to close the slot, providing a major portion of the force needed to move the cutting assembly forward, and providing a lubricant for a block of overburden whose base is formed by cutting the slot and whose movement may be the primary objective in cutting the slot; longitudinal seals extending along the length of the support adjacent the cutter bar, which contact the upper and lower slot surfaces, thus centralizing the support and preventing liquids from flowing forward into the newly cut portion of the slot occupied by the cutter bar; and fluid transfer means spaced along the length of the cutting assembly and adapted to adjust the volume of fluid in the clearances on either side of the support, thus controlling the cutting direction. In cases where the slot cutting operation is intended to enable the movement of a block of overburden, the first-mentioned liquid injected through the cutting assembly into the slot will be a high gel strength paste and, in order to improve the system for lubricating the block, a second liquid passage may be mounted in the support adjacent the trailing edge of the support and a low shear resistance liquid is transmitted through the second passage and discharged to the rear of the support and into the interface between the paste and the bottom surface of the slot. The lubrication system may be further improved by fixing a bevel cutting means to the upper surface of the cutter bar at the slot entrance, thus creating a slight upward bevel at what will be the leading edge of the block of overburden in its subsequent movement. The convergent film geometry, thus created near the leading edge of the block, cooperates with an extra-high gel strength paste placed on a ramp in front of the block to maintain a continuous lubricant film as the block moves down the ramp.